check your 2 selected_button variables, you initialized them using a create empty list block and forgot to assign a button component before using the verification procedure
use Do it to debug your blocks, see also tip 4 here App Inventor: How to Learn | Pura Vida Apps
see also Live Development, Testing, and Debugging Tools
Taifun
Trying to push the limits! Snippets, Tutorials and Extensions from Pura Vida Apps by 
Taifun.